Recent advances in optics provide for a method of exposure of materials on a length scale much smaller than previously realized. Such near-field optical methods are realized by placing an aperture or a lens in close proximity to the surface of the sample or material to be exposed. Special methods for positioning control of the aperture or lens are required, as the distance between the optical elements (aperture or lens) and the material surface is extremely small. Betzig and Trautman in U.S. Pat. No. 5,272,330 reported on the use of tapered optical fibers as a means of providing exposures in extremely small areas; exposures of the size of 10 nm in area are now relatively commonplace. In this case, the fiber tip position is maintained to be within some nanometers (typically 10–50) of the target surface. Others (see, for example, the review by Q. Wu, L. Ghislain, and V. B. Elings, Proc. IEEE (2000), 88(9), pgs. 1491–1498) have developed means of exposure by the use of the solid immersion lens (SIL). The SIL is positioned within approximately 0.3 micrometer of the target surface by the use of special nano-positioning technology as in the case of the tapered optical fiber. In the case of the SIL, marks on a data storage medium can be produced much smaller than the size achievable through the use of conventional or classical optics. Such conventional optics are said to be diffraction-limited because the size of the smallest feature in an image is limited by physical diffraction. Exposures produced by means of the SIL or other near-field optical methods can be much smaller in spatial extent than those produced by conventional optical systems and still be readable. Near-field optics has been used to create single marks in recording medium with such reduced spatial extent. With smaller marks on a data storage system, the resulting density of stored information is correspondingly higher.
Vertical cavity surface emitting lasers (VCSELs) based on inorganic semiconductors (e.g. AlGaAs) have been developed since the mid-80's (K. Kinoshita et al., IEEE J. Quant. Electron. QE-23, 882 [1987]). They have reached the point where AlGaAs-based VCSELs emitting at 850 nm are manufactured by a number of companies and have lifetimes beyond 100 years (K. D. Choquette et al., Proc. IEEE 85, 1730 [1997]). With the success of these near-infrared lasers, attention in recent years has turned to other inorganic material systems to produce VCSELs emitting in the visible wavelength range (C. Wilmsen et al., Vertical-Cavity Surface-Emitting Lasers, Cambridge University Press, Cambridge, 2001). There are many potential applications for visible lasers, such as, display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (T. Ishigure et al., Electron. Lett. 31, 467 [1995]). In spite of the worldwide efforts of many industrial and academic laboratories, much work remains to be done to create viable laser diodes (either edge emitters or VCSELs) that produce light output that spans the visible spectrum.
In an effort to produce visible wavelength VCSELs it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems, since organic-based gain materials can enjoy a number of advantages over inorganic-based gain materials in the visible spectrum. For example, typical organic-based gain materials have the properties of low unpumped scattering/absorption losses and high quantum efficiencies. In comparison to inorganic laser systems, organic lasers are relatively inexpensive to manufacture, can be made to emit over the entire visible range, can be scaled to arbitrary size and, most importantly, are able to emit multiple wavelengths (such as red, green, and blue) from a single chip. Finally, organic lasers have a very large gain bandwidth, especially in comparison with inorganic lasers. Over the past number of years, there has been increasing interest in making organic-based solid-state lasers. The laser gain material has been either polymeric or small molecule and a number of different resonant cavity structures were employed, such as, micro-cavity (Kozlov et al., U.S. Pat. No. 6,160,828), wave guide, ring microlasers, and distributed feedback (see also, for instance, G. Kranzelbinder et al., Rep. Prog. Phys. 63, 729 (2000) and M. Diaz-Garcia et al., U.S. Pat. No. 5,881,083). A problem with all of these structures is that in order to achieve lasing it was necessary to excite the cavities by optical pumping using another laser source. It is much preferred to electrically pump the laser cavities since this generally results in more compact and easier to modulate structures.
A main barrier to achieving electrically pumped organic lasers is the small carrier mobility of organic material, which is typically on the order of 10−5 cm2/(V−s). This low carrier mobility results in a number of problems. Devices with low carrier mobilities are typically restricted to using thin layers in order to avoid large voltage drops and ohmic heating. These thin layers result in the lasing mode penetrating into the lossy cathode and anode, which causes a large increase in the lasing threshold (V. G. Kozlov et al., J. Appl. Phys. 84, 4096 (1998)). Since electron-hole recombination in organic materials is governed by Langevin recombination (whose rate scales as the carrier mobility), low carrier mobilities result in orders of magnitude more charge carriers than singlet excitons; one of the consequences of this is that charge-induced (polaron) absorption can become a significant loss mechanism (N. Tessler et al., Appl. Phys. Lett. 74, 2764 (1999)). Assuming laser devices have a 5% internal quantum efficiency, using the lowest reported lasing threshold to date of ˜100 W/cm2 (M. Berggren et al., Nature 389, 466 (1997)), and ignoring the above mentioned loss mechanisms, would put a lower limit on the electrically-pumped lasing threshold of 1000 A/cm2. Including these loss mechanisms would place the lasing threshold well above 1000 A/cm2, which to date is the highest reported current density, which can be supported by organic devices (N. Tessler, Adv. Mater. 19, 64 (1998)).
One way to avoid these difficulties is to use crystalline organic material instead of amorphous organic material as the lasing media. This approach was recently taken (J. H. Schon, Science 289, 599 (2000)) where a Fabry-Perot resonator was constructed using single crystal tetracene as the gain material. By using crystalline tetracene, larger current densities can be obtained, thicker layers can be employed (since the carrier mobilities are on the order of 2 cm2/(V−s)), and polaron absorption is much lower. Using crystal tetracene as the gain material resulted in room temperature laser threshold current densities of approximately 1500 A/cm2.
An alternative to electrical pumping for organic lasers is optical pumping by incoherent light sources, such as, light emitting diodes (LEDs), either inorganic (M. D. McGehee et al. Appl. Phys. Lett. 72, 1536 [1998]) or organic (Berggren et al., U.S. Pat. No. 5,881,089). This possibility is the result of unpumped organic laser systems having greatly reduced combined scattering and absorption losses (˜0.5 cm−1) at the lasing wavelength, especially when one employs a host-dopant combination as the active media. Even taking advantage of these small losses, the smallest reported optically pumped threshold for organic lasers to date is 100 W/cm2 based on a wave guide laser design (M. Berggren et al., Nature 389, 466 (1997)). Since off-the-shelf inorganic LEDs can only provide up to ˜20 W/cm2 of power density, it is necessary to take a different route to avail of optically pumping by incoherent sources. Additionally, in order to lower the lasing threshold it is necessary to choose a laser structure that minimizes the gain volume; a VCSEL-based micro-cavity laser satisfies this criterion. Using VCSEL-based organic laser cavities should enable optically pumped power density thresholds below 5 W/cm2. As a result practical organic laser devices can be driven by optically pumping with a variety of readily available, incoherent light sources, such as LEDs.
There are a few disadvantages to organic-based gain media, but with careful laser system design these can be overcome. Organic materials can suffer from low optical and thermal damage thresholds. Devices will have a limited pump power density in order to preclude irreversible damage to the device. Organic materials additionally are sensitive to a variety of environmental factors, like oxygen and water vapor. Efforts to reduce sensitivity to these variables typically result in increased device lifetime.
One of the advantages of organic-based lasers is that since the gain material is typically amorphous, devices can be formed inexpensively when compared to lasers with gain materials that require a high degree of crystallinity (either inorganic or organic materials). Additionally, lasers based upon organic amorphous gain materials can be fabricated over large areas without regard to producing large regions of single crystalline material; as a result they can be scaled to arbitrary size resulting in greater output powers. Because of their amorphous nature, organic-based lasers can be grown on a wide variety of substrates; thus, materials such as glass, flexible plastics, and Si are possible supports for these devices. Thus, there can be significant cost advantages as well as a greater choice in usable support materials for amorphous organic-based lasers.
A problem of the prior art data storage is that it relies on a single-track method for creating and retrieving the data and hence has limited access times. Multi-channel means for optical recording using arrays of laser have been proposed to overcome the single channel access limitation. Gupta, in U.S. Pat. No. 5,195,152, describes an apparatus for recording onto and reading from an optical data storage disk with an array of diode lasers. Channel wave guides are used to deliver light from the array of diode laser sources to an optical imaging system. Light from the edge-emitting diodes is coupled to each individual channel wave guide. A multi-channel read system with channel wave guides bringing the signal light reflected from the storage medium to a plurality of detectors is also described. A system for tracking and focus control is included in this apparatus. Zavislan et. al. further describe a multi-channel optical head and data storage system in two separate U.S. Pat. Nos. 5,353,273 and 5,537,617. The multi-channel optical head is employed as part of data read system; the multi-channel optical system is described as an integrated optical read-channel fabricated on a planar wave guide structure. All such systems require the precision alignment of the array of edge-emitting diode laser sources with respect to the planar wave guide structure.
VSCELS are well suited for multi-channel optical head and data storage systems as the multiple laser sources can be patterned onto a single chip and because the light is emitted normal to the chip surface with high beam quality. This facilitates the alignment and delivery of the laser read/write light beam through the optical system and onto the recording medium surface. Gelbart describes such a system in U.S. Patent Application Publication 2002/0136136 A1. In this publication a semiconductor VCSEL array is used to illuminate an optical recording medium for data reading or writing. Conventional optical system components are described in this publication, such as micro-lens arrays used in combination with the linear VCSEL array. Furthermore, width modulation of the written data marks on the surface of the data storage medium is proposed as a means to increase data storage density. This system and other systems read or write data marks on the storage medium whose size is limited by optical diffraction as mentioned above. Near-field optical methods offer the opportunity to achieve smaller marks sizes and thereby even higher optical data storage densities. Furthermore, the use of organic micro-cavity lasers or organic VCSELs enables multi-wavelength systems to be fabricated at low cost